The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it is described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present disclosure.
An internal combustion engine (ICE) may use a sequential turbocharger to increase torque output by delivering additional air into cylinders of the ICE. The sequential turbocharger may be a two-stage turbocharger system. The sequential turbocharger may include a high-pressure turbocharger and a low-pressure turbocharger that are arranged in series. The high-pressure turbocharger may include a high-pressure turbine and a high-pressure compressor. The low-pressure turbocharger may include a low-pressure turbine and a low-pressure compressor.
The high-pressure turbine and the low-pressure turbine may be controlled separately to adjust flow of exhaust gas. The exhaust gas flows through the high-pressure turbine and then through the low-pressure turbine when the engine is operating at low speeds, i.e., speeds less than a predetermined speed. The exhaust gas may bypass the high-pressure turbine and flows through the low-pressure turbine when the engine is operating at high speeds, i.e., speeds greater than or equal to the predetermined speed. The flow of exhaust gas through the turbines can reduce response time of turbocharger boost, referred to as turbo lag.
The turbo lag may be controlled using a bypass valve (BPV) and a variable geometry turbine (VGT). The high-pressure turbine may include the BPV and the VGT. The VGT has a set of movable vanes that are used to control pressure of the exhaust gas flowing through the VGT. The flow of exhaust gas rotates the vanes. Adjusting the openings of the vanes may adjust the acceleration rate of the VGT. The acceleration rate of the VGT tends to increase as the vanes are closed. Boost pressure that is provided to the high-pressure compressor in the high-pressure turbocharger is increased when the acceleration rate is increased. The vanes of the VGT are gradually opened to reduce speed of the VGT as the boost pressure increases. Reducing speed of the VGT prevents the boost pressure that is provided to the high-pressure compressor from exceeding a predetermined level. The BPV may open to redirect the flow of exhaust gas through the BPV when the VGT is operating at high speeds. The BPV opens to relieve the boost pressure of the VGT before the boost pressure exceeds the predetermined level.